Wireless communication devices (direct conversion wireless devices) that include a direct quadrature modulation/demodulation function have been known in which frequency conversion is performed once to convert a baseband (BB) signal into a radio frequency (RF) signal and transmit the RF signal, and a received RF signal is converted into a BB signal by performing frequency conversion (inverse frequency conversion) once. This type of wireless communication device does not need an intermediate frequency (IF) stage for processing IF signals, which gives the wireless communication device an advantage in that an increase in hardware scale is averted.
However, a quadrature modulator/demodulator of this type of wireless communication device cannot avoid in-phase and quadrature imbalance (IQ imbalance) in frequency conversion (quadrature modulation) and inverse frequency conversion (quadrature demodulation) due to the incompleteness of hardware (more strictly, analog hardware components). IQ imbalance is, in other words, interference between an I (in-phase) channel and a Q (quadrature phase) channel in a quadrature modulator/demodulator which is caused by gain imbalance between the I channel and the Q channel and by a quadrature error between the I channel and the Q channel. IQ imbalance occurs particularly frequently when a high frequency such as a frequency in the millimeter band is used for communication, when a broadband signal is handled, or when inexpensive components are employed.
IQ imbalance that occurs in a quadrature modulator/demodulator is expressed as the respective amplitude deviations of the quadrature modulator/demodulator's I channel and Q channel (ati, atq, ari, arq), and the respective phase deviations of the I channel and the Q channel (+φ/2, −φ/2, +ψ/2, −ψ/2) (see FIG. 1). Those imbalances cause interference between frequency components that are symmetric to each other with respect to a center frequency (Fc) of a carrier (carrier wave). In other words, image signals are generated at axisymmetric frequencies (see FIG. 2). The signal quality is thus deteriorated.
In order to avoid this signal quality deterioration, various methods of compensating (correcting) IQ imbalance have been proposed. The proposed methods include a first type of related art for correcting IQ imbalance in a wireless communication device, which removes interference by transmitting from a transmitter side a training signal that is known to a receiver side, and then correcting IQ imbalances of the transmitter and the receiver and the channel characteristics of a propagation environment at once based on a change in the training signal that is observed on the receiver side.
With the method of the first type of related art, the training signal transmitted has been affected by the transmitter side IQ imbalance, and hence, even if an ideal receiver is provided, the influence of the interference cannot be removed unless a correction is made on the receiver side as well.
The proposed methods also include a second type of related art in which correction coefficients for separate corrections on a transmission side and a reception side (separate corrections in a transmission part and in a reception part) are calculated to correct characteristics, and a signal to be transmitted is processed to approach to its ideal form.
To give a more detailed description, in the second type of related art, a path along which a transmission signal is fed back to the reception side is provided, and the path is used when the correction coefficients are calculated to feed back a training signal sent from the transmission side so that the training signal is received on the receiver side. Calculating the correction coefficients also involves IF sampling which is conducted in an analog/digital (A/D) converter by shifting the oscillation frequency of a local oscillator. Signals sampled by IF sampling undergo quadrature demodulation in a digital circuit area. A digital circuit operates as an ideal receiver because quadrature demodulation in a digital circuit is free from the influence of IQ imbalance which is unique to analog circuits.
Therefore, in the second type of related art, an imbalance coefficient that takes into account only the influence of the transmission side IQ imbalance can be calculated by analyzing the received training signal. The calculated transmission side imbalance coefficient is used to correct the IQ imbalance on the transmission side. After the transmission side correction, the training signal is transmitted again and, this time, the reception side local oscillator is operated at a normal oscillation frequency to perform baseband sampling in a normal manner. Thereafter, the received training signal is analyzed to calculate an IQ imbalance correction coefficient for the reception side, and the IQ imbalance on the reception side is corrected with the calculated coefficient.
The method of the second type of related art can remove the influence of IQ imbalance on a transmission signal which is a problem of the first type of related art, but requires an A/D converter that has a band twice or more wider than normal (four times or more wider than the baseband frequency band according to the sampling theorem), in order to execute IF sampling on the reception side. In addition, although capable of removing the influence of IQ imbalance, the second type of related art has difficulties in removing amplitude frequency deviations and phase frequency deviations that are present in intervening analog parts (for example, a low pass filter LPF).
The following are related arts to the invention.    [Patent document 1] Japanese Patent Laid-Open Publication No. JP 2007-60106    [Patent document 2] Japanese Patent Laid-Open Publication No. JP 2008-167057    [Patent document 3] Japanese Patent Laid-Open Publication No. JP 2008-263585    [Patent document 4] Japanese Patent Laid-Open Publication No. JP 2005-527152    [Non-patent document 1] Kamata Hiroyuki, Sakaguchi Kei, and Araki Kiyomichi, “Effects of IQ Imbalance and Effective Compensation Scheme on the MIMO-OFDM Communication System”, Technical report of IEICE, WBS 2004-57, 2005    [Non-patent document 2] Tanabe Yasuhiko, Egashira Yoshimasa, and Sato Kazumi, “A study on IQ Imbalance Correction Scheme for MIMO-OFDM Systems”, Technical report of IEICE, RCS 2006-272, 2007